Those early rubber-hose cartoon characters, with their basic facial designs altered only by the shape of their ears and, occasionally, noses... it can be pretty hard to tell what kind of creature they're supposed to be. Indeed, they made a running joke of it many years later in Animaniacs, where people often wonder what species the Warners are, with their late-20s-early-30s inspired designs.
The Warner Brothers studio's first character, Bosko, fits the usual design pattern - white face, black nose, solid black eyes, and black... hair? covering the rest of his head. But he was apparently meant to be a human character. His ears, the usual species giveaway, look like simplified forms of human ears, compared to the large black things protruding from the tops of Oswald the Lucky Rabbit, Mickey Mouse or Foxy's heads. In fact, he was meant to be a *black* human.
Which seems quite a bizarre claim to make about a white-faced character, but when Harman and Ising took the character to MGM, they made him (and Honey) obviously dark-skinned human characters. I suppose when you think of the stereotyped depictions of black people that were common at the time, Bosko's white face could be seen as comprising large white eyes and large white lips... with the black portion behind his face being his skin.
Certainly, when Bosko and Honey were brought back for an appearance on Tiny Toon Adventures, they were redesigned to have floppy black ears atop their head... making them a more indistinct species like the later Warners on Animaniacs. So, to the people who made that episode at least, there was something controversial about their original appearance.
However, Bosko doesn't really read as a "blackface" caricature. Harman and Ising, and their staff, probably forgot that's what he was meant to be in the first place, or discarded the idea. Take a look at the final gag from this cartoon:
A bomb goes off in Bosko's face, covering it with... bomb dust or whatever, and transforms him *into* a blackface character, announcing "Mammy!" So if he had to be turned into a blackface character, what is he the rest of the time?